The Basics of Blasting Vibrations and Noise
Highway construction projects; placement of sewer, water, gas and electrical services; residential and commercial building projects; and mining frequently require the use of explosives. In these applications, rock must be broken loose from the ground and blasting is the only practical way to accomplish this task. Unfortunately, the public has many misconceptions about the use of explosives.Blasting agents, a type of explosives, are preferred in most situations because they are economical and safe. ANFO, a mixture of fuel oil and ammonium nitrate (the same compound as commercially-available fertilizer), is the blasting agent most commonly used. Dynamite is seldom used today, and similar high-energy explosives are needed only in small quantities.
Explosives manufacturers have done extensive research on ways to improve their products, with the most important goals being enhanced safety and efficiency. As a result, it now takes only about 8 ounces of explosives to break loose a ton of rock (an impressive 4,000 to 1 ratio).
The first step in using explosives to break rock is drilling a series of holes, usually 3-6 inches in diameter, in which to place the blasting agent. Careful planning goes into determining a precise pattern for these holes since this is a major factor in achieving desirable results. Before a blasting agent is placed in a hole, it is preceded by a small booster charge.
A booster provides just enough energy to detonate the blasting agent. Boosters in each hole are detonated individually by blasting caps that have built-in time delays. Using delays just thousands of a second apart is enough to greatly disperse the energy released by the total amount of explosives involved. To an observer, a blast seems to happen instantaneously. What actually takes place, however, is a rapid progression of smaller explosions.
More than 90 percent of a blast's energy is consumed as it fractures the rock. This efficiency is achieved by adjusting the amount of explosives detonated per delay interval, which is also important in minimizing ground vibration. It's possible to have a 100-hole blast, for example, create the same ground vibration as a 50-hole blast, even though there's twice as much explosives involved in the 100-hole blast.
Only the remaining energy not "used up" as it breaks rock is capable of causing ground vibration. Ground vibration caused by any source rapidly decreases as the distance from the source increases. Advance planning of the amount of explosives used and the time delays employed remarkably effective in controlling the amount of vibration that will occur from a blast. However, quarries and other users of explosives often use seismographs that measure ground vibration to verify their predictions. These sensitive instruments are placed on the surface of the ground, where the most intense vibration from blasting occurs.
Studies by government agencies, universities and engineering firms have investigated the effects of ground vibrations on residential structures. Criteria derived from these studies are an invaluable reference tool. Companies using explosives in populated areas typically establish a self-imposed margin of safety to insure that vibrations are well below intensities that create slight cosmetic damage to even the weakest type of structure.
Research has shown that the first evidence of damage to a structure from vibration is hairline cracks in plaster. This component of a home's construction is brittle and has the lowest structural strength. Drywall and sheetrock would be next, followed by brick and mortar. Poured concrete is the strongest of all. it can withstand high vibration levels with no effects. Blast vibrations intense enough to crack a foundation or patio would cause profound effects on other building components.
It was pointed out earlier that blast vibrations are the most intense on the earth's surface. Therefore, underground objects such as wells, pipelines, swimming pools and septic tanks are subjected to less vibration than objects sitting on top of the ground. When explosives are detonated, vibrations measured 150 feet below the surface, for example, are typically 15 percent of the intensity at ground level. It's a common misconception that well casings, because they are underground, are easily damaged by blasting vibrations, but overwhelming research and field evidence has proven that there is no justification for this concern. It is possible for people with shallow wells to notice a slight, temporary increase in water turbidity after a blast. This is caused by fine sediment becoming dislodged from around the well casing, which may occur a low vibration levels.
Research has also shown that the denser a material is (rock compared to soil, for example), the less efficient it is at transmitting vibrations. A homeowner with a foundation embedded in the same shelf of rock being mined as a nearby quarry operation has no reason to be concerned that the home will experience more vibration than other homes with foundations in soil that are the same distance from the blasting.
"Airblast" is a term commonly used to describe excess energy that is released into the atmosphere as a result of explosives detonation. A more correct term for it is "overpressure". Although this pressure change traveling through the air transmits noise from a detonation, most of the energy is below the frequency range of human hearing. The lower frequency air pressure may cause windows to rattle, which is noise caused by the glass shifting in a slightly loose window frame.
Natural phenomena that influences overpressure are wind and variations in temperature at different heights in the atmosphere. Therefore, the effects of overpressure may be less or more noticeable in different days. Although overpressure is often what causes people to be aware of the use of explosives at a quarry or construction project, it has virtually no likelihood of damaging a structure (including windows).
Overpressure from a detonation, by the way, is transmitted through the air about 10 times slower than vibration is transmitted through the ground. Although traveling faster, ground vibration dissipates more rapidly than overpressure. This means people can often hear a detonation at greater distances than they can detect ground vibration from it.
Even when blasting practices are followed that produce effects far below that which may cause structural damage, it is still possible for a detonation to be noticed by people some distance away. The reasons for this are that: (1) Vibration is perceptible to humans at surprisingly low levels and (2) Air pressure changes, such as overpressure, are dissipated relatively slowly. The noise of a blast, rattling windows or a slight vibration may startle some people, but it is extremely rare that any type of damage is occurring.
It is possible for a house's structural components, or some of the items it contains, to harmlessly "resonate" at nearly the same vibration frequency that is caused by blasting. This may cause slightly more noticeable effects, such as rattling dishes. Since the duration is so short, it does not damage the structure or anything within it. Granted, it is conceivable that a precariously balanced dish on a wall shelf may fall in such a circumstance. However, it is more likely to be dislodged by other activities in the home.
People are usually surprised to learn that slamming doors, truck traffic, children running and hammering nails produce much greater vibration effects in a home than those produced by the nearby use of explosives. These activities do not cause as much concern as blasting because they are expected and routine.
Environmental forces also contribute a surprising amount of stress to a home. These forces include soil settling, wetting/drying of soil (causing expansion and contraction around the foundation), humidity, temperature changes and wind. The Effects of Vibrations and Environmental Forces: A Guide for the Investigation of Structures states that environmental strains might be 50 to 100 times greater than the level of blasting vibration that is noticed by humans. Sooner or later, every home will show the effects of natural elements. Common effects are cracked plaster and concrete, which homeowners sometimes mistakenly attribute to blasting.
Explosives must be used to provide some of the products that are necessary for our society. The crushed stone industry, for example, must use explosives in the production of construction aggregates. These aggregates are needed for every residential, commercial and infrastructure project. While using explosives, it's always a high priority for our industry to take whatever precautions are necessary to protect the public's safety and property.
For more information about the use of explosives, go to www.explosives.org